Exposure system, semiconductor device, and method for fabricating the semiconductor device

ABSTRACT

In order to link a defect inspection process before forming contact holes with an exposure process for forming the contact holes, a position (physical coordinates) of a defect on a wafer is stored, the defect having been detected in the defect inspection process before forming the contact holes, an exposure (dummy exposure) is performed under the condition that no contact hole is formed on the above-mentioned position. In this method, no contact hole is formed in the region having the defect therein, the cell is considered as a defective one, yet a word line (control gate) and a bit line are not short-circuited through the contact hole, and makes it possible to avoid the short-circuiting by only applying a redundancy to the bit line as conventionally employed.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of International Application No.PCT/JP2004/012477, filed Aug. 30, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to semiconductor devices and methodsfor fabricating thereof, and more particularly, to a fabricatingtechnique of a non-volatile semiconductor memory device that hasimproved redundant efficiency of a flash memory of a NOR type.

2. Description of the Related Art

A flash memory, which is a non-volatile semiconductor memory device, hasboth features of a RAM (Random Access Memory) on which data isrewritable and ROM (Read Only Memory) on which the data is retainableafter the power is off. Generally, a smallest memory unit in a memorydevice is known as cell, and one cell stores one bit. SRAM or DRAM iscomposed of a single cell made of multiple elements. On the other hand,a single cell in the flash memory is composed of only one transistor,which is a smallest number of the elements. Therefore, it is possible toreduce the cost. The aforementioned single cells are grouped into onesector (block), and a memory region is formed by assembling theaforementioned sectors. Data erase is performed in the memory region ona sector basis (on a chip basis).

The flash memory is categorized into a NAND type and a NOR type. A flashmemory of the NAND type includes one data line directly connected by8-bit or 16-bit of the memory cell, and employs Fowler-Nordheim tunneleffect with the use of whole surfaces of a silicon substrate and afloating gate in both writing and erasing. In contrast, the flash memoryof the NOR type includes memory cells respectively connected in parallelto one data line, and utilizes a hot electron in writing and theFowler-Nordheim tunnel effect in erasing.

FIG. 1 is a schematic cross-sectional view illustrating a connectionbetween the cell in the flash memory of the NOR type and the bit line. Aflash memory cell of the NOR type 10 is formed of a semiconductorsubstrate 11, a floating gate 12, a word line 13, a contact portion 14(composed of contact holes and conductive members therein), an insulatorlayer 17, and a bit line 15. The floating gate 12 and the word line 13are formed inside the insulator layer 17 provided on the semiconductorsubstrate 11. The contact portion 14 is provided in the insulator layer17. The bit line 15 establishes contact with the semiconductor substrate11 (specifically, contact with an impurity diffused layer) through thecontact portion 14. With respect to each cell 10 in the flash memory ofthe NOR type, whether or not the information is stored can be determinedby storing an electron in the floating gate 12, the electron having beeninjected by applying voltage from the word line 13.

In a fabricating process of the cell having the above-mentionedstructure, possibly there arises a problem in operation caused resultingfrom an electric short circuit between the word line 13 and the contactportion 14 due to a defect 16. However, even if a redundancy is appliedto the bit line in the problem in operation, the word line 13 stillshort-circuits the contact portion 14 connected to the bit line 15 towhich the redundancy is applied. This prevents the word line 13 frombeing supplied with sufficient voltage at the time of writing, reading,and erasing the data, and the problem in operation cannot be solved.

This time, even if the redundancy is applied to the word line, onlyerase operation is performed, yet write operation is not performed onthe cell provided on the redundant word line 13 and connected to anirredundant bit line 15. This results in an affect in reading othercells connected to the same bit line 15, because a threshold voltage ofthe aforementioned cell becomes negative.

FIG. 2 is a view illustrating a circuit configuration of the flashmemory of the NOR type. As shown in this figure, one memory cell 10 isconnected between the word line 13 and the bit line 15. Any one of thememory cells 10 is conductive, the potential of the bit line 15decreases.

In FIG. 2, assuming that there is an electric short-circuit between aword line 13-1 and a contact hole provided on a cell 10-1, which islocated on an intersection of the word line 13-1 and a bit line 15-1,and then the redundancy is applied to the word line 13-1 and the bitline 15-1 to fix the short-circuiting. The flash memory of the NOR typeis erased on a sector (memory block) basis, and accordingly, the eraseoperation is performed on not only the cell 10-1 but also other cellsconnected to the redundant word line 13-1 and the redundant bit line15-1 simultaneously. On the other hand, the write operation is performedon the word line and the bit line selected, the cells connected to theredundant word line 13-1 and the redundant bit line 15-1 cannot beselected and the write operation cannot be performed. Even if the cellsconnected to the redundant word line 13-1 and the redundant bit line15-1 can be selected, the sufficient voltage cannot be supplied to theword line and the write operation cannot be performed.

The threshold voltage of the cell, in which only the erase operation isperformed and the write operation is not performed, gradually decreasesand becomes negative, and then the cell lets a current flow through in astate of the gate voltage of 0 V. On the flash memory of the NOR type,the current flowing through the bit line is detected as a cell current,yet the currents of the cells connected to irredundant bit lines(15-0,15-2, and 15-3) are affected by the cell currents connected to theredundant word line 13-1. This makes it impossible to read the selectedcell current properly.

As described above, the redundancy cannot be applied to the problem inoperation induced by the electric short-circuit between the word line 13and the contact portion 14, and the problem cannot be solved.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has objects of providing a method for fabricating a non-volatilesemiconductor memory device that can solve a problem in operation causedresulting from a defect located between a word line and a contact holeonly with the use of redundancy of a bit line, and thereby improving ayield ratio of manufacturing.

According to an aspect of the present invention, preferably, there isprovided an exposure system including inspection equipment that detectsa defect on a semiconductor wafer, a controller that stores physicalcoordinates of the defect on the semiconductor wafer, and an exposuredevice that exposes the semiconductor wafer under an exposure conditioncontrolled by the controller so that no contact hole is formed in aregion corresponding to the physical coordinates of the defect. On theaforementioned exposure system, the exposure device may expose theregion under a defocused condition or irradiates no beam onto theregion. On the aforementioned exposure system, the exposure device is anelectron beam exposure device of a scanner type.

According to another aspect of the present invention, preferably, thereis provided a method of fabricating a semiconductor device comprisingthe steps of detecting a defect on a semiconductor wafer in advance offorming contact holes, storing physical coordinates of the defectdetected, exposing the semiconductor wafer to form the contact holes sothat no contact hole is formed in a region corresponding to the physicalcoordinates of the defect. The aforementioned step of exposing mayexpose the semiconductor wafer under a defocused condition or irradiatesno beam onto the region. The aforementioned step of exposing forms thecontact holes so as to make a memory cell arrangement of a NOR type.Preferably, the above-mentioned method may further include a step ofreplacing a bit line that includes the region having the defect with aredundant bit line.

According to another aspect of the present invention, preferably, thereis provided a semiconductor device including a semiconductor substrate,an insulator layer formed on the semiconductor substrate, bit linesformed on the insulator layer, contact holes that are formed in theinsulator layer between the bit lines and the semiconductor substrate,and floating gates and word lines formed within the insulator layer. Thesemiconductor device having a regular arrangement of contact holes whicharrangement may include a position at which no contact hole is formed.On the aforementioned semiconductor device, the bit lines may include aredundant bit line located at the position at which no contact hole isformed.

According to the present invention, no contact hole is formed in theregion in which there is a defect, and the cell is considered as adefective one. Accordingly, the word line (the control gate) and the bitline is not short-circuited through the contact hole, and it is possibleto avoid short-circuiting, by applying a redundancy on the bit line asconventionally employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a connectionbetween a cell in a flash memory of a NOR type and a bit line;

FIG. 2 is a view illustrating a circuit configuration of the flashmemory of the NOR type;

FIG. 3 is a block diagram of an exposure system illustrating a linkagebetween a defect inspection equipment inspecting the defect beforeforming contact holes and an exposure device of a scanner type exposingfor forming the contact holes;

FIG. 4 is a flowchart exemplifying a part of a sequence of processesstarting from the above-mentioned defect inspection process through anexposure process;

FIG. 5A is a first view illustrating component parts of a transistor tobe formed in the process in the flowchart shown in FIG. 4;

FIG. 5B is a second view illustrating the component parts of thetransistor to be formed in the process in the flowchart shown in FIG. 4;

FIG. 5C is a third view illustrating the component parts of thetransistor to be formed in the process in the flowchart shown in FIG. 4;

FIG. 5D is a fourth view illustrating the component parts of thetransistor to be formed in the process in the flowchart shown in FIG. 4;

FIG. 6A is a top plan view illustrating how to avoid short-circuitingbetween the word line and the contact hole without forming the contacthole; and

FIG. 6B is a top plan view illustrating how to avoid short-circuitingbetween the word line and the contact hole without forming the contacthole.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A bottom cause of the above-mentioned problem in operation comes from adefect (for example, a particle adhered on a wafer (semiconductorsubstrate)) occurred in a manufacturing process of the non-volatilesemiconductor memory device, and the defect becomes a problem in formingnormal contact holes in a consequent process. The electric short-circuitbetween the contact hole and the word line makes it impossible to applythe redundancy. However, the aforementioned defect can be detected by adefect inspection in advance of a process of forming the contact holes.If there is no contact hole formed on a position of the defect, the wordline and the bit line are not short-circuited through the contact hole.It is therefore possible to avoid short-circuiting by applying theredundancy on the bit line as conventionally employed. In other words,by replacing “a defect of the particle” with “a defect with no contacthole”, the conventional redundant method is applicable and the problemin operation can be solved.

On this account, in the present invention, a defect inspection processbefore forming the contact holes is linked with an exposure process forforming the contact holes. Specifically, a position (physicalcoordinates) of the defect on a wafer, which has been detected in thedefect inspection process before forming the contact holes, is stored,and an exposure (dummy exposure) is performed so that no contact hole isformed on the aforementioned position. Here, the dummy exposure denotes,for example, the exposure under a defocused condition of avoidingforming the contact hole, or an “empty exposure” that does not expose(does not irradiate no beam onto) the region. The dummy exposure will bedescribed as the exposure under the defocused condition forsimplification; however, the present invention is not limited to theexposure under the defocused condition. As described, there is nocontact hole formed on the position (physical coordinates) of the defecton the wafer that has been detected in the defect inspection processbefore forming the contact holes, in accordance with the presentinvention.

The position of the defect can specified by a pattern recognition thatcompares images of adjacent circuit patterns, and in addition, not onlythe physical coordinates of the defect but also a spatial spread (size)thereof can be measured at the time of the pattern recognition. Also, anexposure device embodying the present invention is a “scanner type” (forexample, an EB exposure device), which can set exposure conditionsaccording to every contact hole. If there is a defect in a region inwhich a contact hole is formed, the defocused exposure condition isintentionally configured to perform the dummy exposure instead of asubstantial exposure.

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

FIG. 3 is a block diagram of an exposure system illustrating a linkagebetween a defect inspection equipment inspecting the defect beforeforming the contact holes and an exposure device of a scanner typeexposing for forming the contact holes. In FIG. 3, a reference numeral310 represents a surface inspection equipment and a reference numeral320 represents an exposure device of a scanner type, both of whichincludes stages 311 and 321 to mount a wafer 301 thereon. Information onthe physical coordinates of a defect (for example, a particle) 302,which has been detected on the wafer 301 in the defect inspection withthe surface inspection equipment 310 before forming the contact holes,is transmitted to and stored in a controller (for example, a personalcomputer) 330. The aforementioned surface inspection is carried out by ageneral method. For example, laser beams are entered obliquely into thesurface of the wafer 301, scattered beams are monitored with a detectingportion 312, and the pattern of the image is analyzed according tosignals. In addition, the size of the defect is obtainable by analyzingthe strength of the scattered beams, in the image analysis.

The exposure has been performed in order to form the contact holes onthe wafer that has completed the surface inspection. As described above,if the contact hole is formed on a coordinate position of the defect 302on the wafer 301, the problem cannot be solved only by the redundant bitline. Therefore, the controller 330 transmits a signal of setting thedefocused exposure condition to an exposure portion 322 when theexposure is performed in a region on the wafer 301 corresponding to thedefect 302 with the use of the exposure device 320. In addition, asdescribed above, another signal is transmitted to the exposure portion322, indicating that the region on the wafer 301 corresponding to thedefect 302 is not exposed.

FIG. 4 a flowchart exemplifying a part of a sequence of processesstarting from the above-mentioned defect inspection process through theexposure process. FIGS. 5A through 5D are views illustrating componentparts of a transistor formed by the processes in the flowchart shown inFIG. 4.

After a second gate etching process (step S201) is completed, the waferis inspected whether or not there is a defect thereon with the defectinspection equipment (step S202). If the defects are detected, theinformation on the physical coordinates corresponding to the positionsof the respective defects are transmitted to and stored in thecontroller (step S203). Then, a deposition is performed to formsidewalls (step S204). After this process, the defect inspection isperformed same as described above (step S205), and the controller storesthe result (step S206).

For example, as shown in FIG. 5A, the floating gate 12 and a controlgate 13 a are provided on the surface of the substrate 11 having asource 11 a and a drain 11 b formed thereon. Sidewalls 17 are providedon the sides of the gates. Originally, the contact holes are formed inbetween the sidewalls 17 on the gate sides. However, if there is thedefect 16 in the corresponding region, it is necessary not to form thecontact hole in the aforementioned region. Therefore, the information onthe physical coordinates that has been detected in the step S205 istransmitted to and stored in the controller (step S206), and then isused as the position that should be defocused in a focus adjustment inthe subsequent exposure process. As described above, if the “emptyexposure” is employed instead of the defocused exposure, the informationon the physical coordinates of the defect 16 is used as the position onwhich the exposure is not performed.

After a mask process for introducing impurities into the source 11 a andthe drain 11 b (step S207) and an ion implantation (step S208), adeposition process is performed to bury the gate (step S209). Then, aphotoresist mask is formed on the aforementioned buried layer to formthe contact hole (step S210).

FIG. 5B shows a photoresist mask 18 that has been exposed not to formthe contact hole on the position of the defect 16. The photoresist mask18 is formed by exposing the photoresist evenly applied on the gateburied layer 19 to have an opening only in the region corresponding tothe position on which the contact hole is to be formed. A regionrepresented by a reference numeral 18 a in FIG. 5B corresponds to theregion in which the opening is to be provided for forming the contacthole. However, there is the defect 16 corresponding to theaforementioned position. Therefore, the controller transmits theinformation on the physical coordinates of the defect to the exposuredevice not to form the opening on the corresponding position (stepS211), and the exposure device performs the dummy exposure so that thecontact hole may not be formed on the region having the defect thereinunder the defocused exposure condition, according to the information.

Etching is performed for forming the contact holes with theabove-mentioned photoresist mask 18 (step S212 and FIG. 5C), and thecontact is established to form the bit line 15 (step S213 and FIG. 5D).

FIGS. 6A and 6B are top plan views illustrating how to avoidshort-circuiting between the word line and the contact hole withoutforming the contact hole. In the conventional method (FIG. 6A) offorming the contact portion 14 regardless of existence of the defect 16,the contact portion 14 formed in the region of the defect 16short-circuits the word line 13 and the bit line 15, and the problemcannot be solved by only applying the redundancy to the bit line. Incontrast, according to the method of the present invention (FIG. 6B),the contact region 14 (more specifically, the contact hole) is notformed in the region of the defect 16. The cell is regarded as adefective one, yet the word line (the control gate 13 a) and the bitline 15 are not short-circuited through the contact portion 14. It istherefore possible to solve the problem by only applying the redundancyto the bit line as employed conventionally. In other words, by replacing“a defect of the particle” with “a defect with no contact hole”, theconventional redundant method is applicable and the problem in operationcan be solved.

A configuration of the flash memory shown in FIG. 6B is described withthat shown in FIG. 1. The flash memory includes the semiconductorsubstrate 11, the insulator layer 17 formed on the semiconductorsubstrate 11, the bit line 15 formed on the insulator layer 17, thecontact portion 14 establishing a contact between the bit line 15 formedinside the insulator 17 and the semiconductor substrate 11, the floatinggate 12 formed in the insulator layer 17, and the word line 13. Thecontact portion 14 has a regular arrangement, and a portion (the portioncorresponding to a reference numeral 16), which is irregularly providedin the regular arrangement, is also included. The redundant process isapplied to the bit line on which the contact portion is not formed.According to the present invention, only applying the redundancy to thebit line can cope with the problem, and makes it possible to provide thesemiconductor device that solves the problem in operation effectively

Here, the defect inspection is performed twice, that is, after thesecond gate etching process and after the sidewall deposition process.However, appropriate number of timed may be provided according to theprocess design of the device to be manufactured.

The present invention makes it possible to provide a method offabricating a non-volatile semiconductor memory device in whichredundancy effects are improved and provide an exposure system thatenables the method.

Although a few preferred embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these embodiments without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. An exposure system comprising: inspection equipment that detects adefect on a semiconductor wafer; a controller that stores physicalcoordinates of the defect on the semiconductor wafer; and an exposuredevice that exposes the semiconductor wafer under an exposure conditioncontrolled by the controller so that no contact hole is formed in aregion corresponding to the physical coordinates of the defect.
 2. Theexposure system as claimed in claim 1, wherein the exposure deviceexposes the region under a defocused condition or irradiates no beamonto the region.
 3. The exposure system as claimed in claim 1, whereinthe exposure device is an electron beam exposure device of a scannertype.
 4. A method of fabricating a semiconductor device comprising thesteps of: detecting a defect on a semiconductor wafer in advance offorming contact holes; storing physical coordinates of the defectdetected; exposing the semiconductor wafer to form the contact holes sothat no contact hole is formed in a region corresponding to the physicalcoordinates of the defect.
 5. The method as claimed in claim 4, whereinthe step of exposing exposes the semiconductor wafer under a defocusedcondition or irradiates no beam onto the region.
 6. The method asclaimed in claim 5, wherein the step of exposing forms the contact holesso as to make a memory cell arrangement of a NOR type.
 7. The method asclaimed in claim 4, further comprising a step of replacing a bit linethat includes the region having the defect with a redundant bit line. 8.A semiconductor device comprising: a semiconductor substrate; aninsulator layer formed on the semiconductor substrate; bit lines formedon the insulator layer; contact holes that are formed in the insulatorlayer between the bit lines and the semiconductor substrate; andfloating gates and word lines formed within the insulator layer, thesemiconductor device having a regular arrangement of contact holes whicharrangement includes a region in which no contact hole is formed.
 9. Thesemiconductor device as claimed in claim 8, wherein the bit linesinclude a redundant bit line arranged in the region in which no contacthole is formed.